How can I mark the seams to UV unwrap a torus?
There are many ways to unwrap a torus (or any mesh). A lot of it depends on your needs and the kind of projection you are after.

One way is to mark one seam that splits the torus horizontally and one that does it vertically:



Then select one of the faces where the seams meet



and press A to deselect and A again to select all faces. That way you will have all of the faces selected but will also have an active face.

Press U to unwrap and select follow active quads



The result should look like this:



Form there on you can scale, assign a texture, etc.